1. Field of the Invention
The invention relates to a process of reforming gasolines, under low pressure, int he presence of at least one catalyst, in which process the heat required for the reaction is brought by a smokes-based heat-carrying fluid coming, for example, from the combustion of air with a hydrocarbon mixture, for example natural gas or refinery gas.
The invention also relates to the apparatus in which said method may be put into practice.
2. Description of the Prior Art
It is known to improve the quality of gasolines, and particularly their octane number, by reforming at about 500.degree. C., in the presence of hydrogen, in several solid catalytic beds comprising precious metals activated by different additives, such as rhenium and iridium for example (Pat. Nos. US-A-4,588,495, FR-B-2,593,324, FR-B-2,597,496, FR-B-2,600,668).
The reforming of gasolines results from an assembly of reactions whose balance is overall endothermic and a drop of the reaction pressure usually occurs as the transformations progress.
The traditional method of reforming gasolines comprises three, and generally four, adiabatic reactors, disposed in series and connected together by two, and generally three, heating ovens, whose role is to heat the reagents and to bring and maintain their temperature thus int he desired reaction temperature range.
The application of anti-pollution standards leads to a reduction of the lead concentration in gasolines and will in the long run result in suppressing organo-plumbic additives in motor car fuels for controlled ignition engines. Such regulations impose on refiners an increase in the severity of the operating conditions of the reforming units so as to satisfy the criteria of quality required for satisfactory operation of the motor vehicle engines: thus refiners are led to reduce more and more the operating pressure of these reforming units. In fact, if the severity of the operating conditions is increased, in particular the octane number of the reformed products, the yield drops and the only means of compensating for such a drop of yield is to operate at a lower pressure.
But such low pressure operation is expensive and R.G. McClung et al have recently shown (Hydrocarbon Processing, Sept 1983, pages 80-84) that such a system cannot actually go down below the limit of 1 MPa without serious drawbacks.
The major obstacle is due in particular to the pressure 20 drop (for example 0.4-0.5 MPa) of the unit constructed from such a reaction system.
Now, the Applicant has found that it is possible to considerably reduce this pressure loss and to carry out reforming at a pressure not exceeding 2 MPa, preferably 1 MPa.
The process of the invention is in particular based on the use of a reaction system in which the heat required for the reaction is brought to the catalytic bed(s) by a smokes (or heating gases) based heat-carrying fluid.
Thus, reforming in accordance with the invention is carried out in one or two reaction zones, one at least of which contains heat exchanging channels or spaces, preferably substantially parallelepipedic, inside which flows the heating smokes-based heat-carrying fluid, as will be explained further on in detail.
By way of indication, one of the hydrocarbon charges to be reformed may generally contain, in volume, 35 to 80% of paraffinic hydrocarbons, 14 to 51% of naphthenic hydrocarbons and 2 to 18% of aromatic hydrocarbons. It is generally formed essentially of gasolines. This charge is for example a naphtha distilling between about 55 and about 225.degree. C.
Reforming in accordance with the invention is carried out on a mixture of said charge and recycled hydrogen (or recycling hydrogen), in a proportion such that the molar ratio of the hydrogen with respect to the charge of hydrocarbons to be processed (H2/HC) is usually between about 1 and 100, and preferably between 2 and 10.
Reforming is carried out at a temperature usually between about 300 and 700.degree. C., preferably between about 400 and 600.degree. C.
Any reforming catalyst known to a man skilled in the art may be used, in the form of grains which are for example substantially spherical.
Supported catalysts will for example be used comprising at least one noble metal of group VIII of the periodic classification of elements, generally platinum, preferably doped with at least one promoter such as indium, germanium, iridium (US-A-2,848,377), rhenium (US-A-3,415,737), tin (US-A-3,700,588). Generally a halogen such as chlorine or fluorine is incorporated in the catalyst (FR-B-2,600,668).
The supports of the catalysts are usually chosen from the oxides of the metals of groups II, III and/or IV of the periodic classification of elements, such for example as magnesium, aluminum, titanium, zirconium, thorium or silicon oxides, taken alone or mixed together or with oxides of other elements from the periodic classification, such for example as boron. Charcoal may also be used. Zeolites or molecular sieves of type X or Y may also be used or of mordenite or faujasite type or of ZSM-5, ZSM-4, ZSM-8 L, etc... type, as well as mixtures of oxides of metals from groups II, III and/or IV with zeolitic material.
Preferably, a support is used formed mainly of alumina, i.e. in which the alumina represents at least 50% by weight with respect to the total weight of the support and preferably at least 80% by weight, and more preferably alumina alone is used.
FIGS. 1 and 2, given by way of examples, illustrate examples of implementing the invention where the reforming takes place in a single reactor (FIG. 1) or in two reactors placed in series (FIG. 2).
The hydrocarbon charge to be processed, preferably desulfurated, for example a mixture of gasolines from direct distillation of crude oil and/or from distillation of thermally or catalytically cracked oil products, is fed through duct 1 of the reforming unit where it is mixed with hydrogen which is brought through duct 3 in a ratio generally about 1 to 100, preferably about 2 to 10 moles of hydrogen per mole of hydrocarbon charge to be processed.
The mixture thus formed which is then in accordance with the invention under a pressure ranging from 0.05 to 2.0 MPa, preferably from 0.05 to 1.0 MPa, is then preferably indirectly pre-heated by hot reaction effluent (coming from duct 6) through the heat exchanger 2 usually to a temperature between about 300 and 700.degree. C., advantageously between about 400 and 600.degree. C.
Said mixture then passes, through duct 4, into the reactor 5 in accordance with the invention and described subsequently, said reactor containing an above defined reforming catalyst.
In reactor 5, the gasolines are reformed, their molecules being at least partially transformed by dehydrogenation, isomerisation, possibly dehydrocyclisation and cracking reactions, into compounds having appreciably improved anti-knock properties.
These reactions result in a large production of hydrogen and a high heat demand. In conventional units with adiabatic reactors, the endothermicity of the reactions causes the temperature to drop rapidly and, in order to continue the reforming operations, it is then necessary to carry out intermediate heating, by feeding the reagents over one or more external ovens.